The present invention is directed to methods and apparatus for voltage measurement with a particle probe without an external trigger signal.
Numerous methods are known in the prior art for acquiring signal curves of periodic test signals with a particle beam. A summary of currently standard test methods is reproduced in the publication "Microelectronic Engineering" 4 (1986), pages 77 through 106, "Electron Beam Testing", by E. Wolfgang, particularly on page 83. A distinction is made between measurements that are carried out in the time domain and measurements that are carried out in the frequency domain. Including in the first, for example, are "Voltage Coding" (also see "Scanning Electron Microscopy" 1975 (part I) Proc. of the 8th Annual Scanning Electron Microscope Symposiuim, Chicago, IIT Research Institute, pages 465 through 471) or "Logic State Mapping" (also see U.S. Pat. No. 4,223,220), whereas what is referred to as a "Frequency Mapping Method" is included in the latter.
In both cases (measurements in the time and frequency domain), a focused primary particle beam is directed onto a specimen surface to be scanned with the use of deflection coils. A detector collects the triggered secondary particles thereby an the interpretation of a resulting secondary signal occurs in a signal chain.
In most measurements in the time domain, it is necessary to have a trigger signal available that is synchronized to a signal in the specimen, for example an integrated circuit (IC). Such a trigger signal is necessary in order to acquire a measured signal from the secondary signal by a sampling method or by a combined sampling and averaging method in a part of a signal chain, for example in a measurement processing arrangement.
Among other things, voltage measurement based on the "Waveform Measurement" method on which the method of the present invention is based is explained in the publication Scanning, Vol. 5, pages 14 through 24 (1983) (especially pages 18 through 20), "Electron Beam Testing: Methods and Applications" by H. P. Feuerbaum. The implementation of this method with the use of a boxcar integrator is set forth in FIG. 10 (page 20) of this publication. The block diagram structure of the boxcar integrator is essentially composed of a phase control unit, of a delay unit, of a gate circuit and of a measurement processing unit. European Pat. No. 00 48 858 also sets forth the structure and the functioning of a boxcar integrator.
Many integrated circuits, however, have asynchronous circuit components whose internal signals cannot have their frequency acquired from applied external signals or there are free running oscillators in the IC whose signals are not externally accessible. In such cases, an externally generated trigger signal is not available.
Asynchronous circuit components such as, for example, free running oscillators, can be currently investigated only with real-time methods. The use of real-time methods, however, is limited to relatively slow signals (see Scanning Electron Microscopy (1982), "IC-Internal Electron Beam Logic State Analysis" by M. Ostrow, E. Menzel, E. Postulka, S. Goerlich, E. Kubalek, pages 563 through 572. Also, obtainable precision is low because of the poor signal-to-noise ratio. The methods usually utilized for fast signals, by contrast, use a combined sampling and averaging principle. A boxcar integrator that works based on the combined sampling and averaging method is, for example, a model 162 of Princeton Applied Research, described in the "Princeton Applied Research Operating and Service Manual". To this end, however, a trigger signal for controlling the measurement processing arrangement in the signal chain is necessary.